Police were monitoring Cinco de Mayo cruising on Harbor Boulevard late Thursday when they saw Leann Renee Arriaga,20, drive through a red light at First Street. When they pulled her over, found an open bottle of alcohol in her car, and performed what’s known as a “Preliminary Alcohol Screening” — a field sobriety test.

The 20-year-old registered a .30 blood-alcohol level. The legal limit is .08, and that’s for someone who has already turned 21. Arriaga turns 21 on Saturday.

“I’ve never seen anybody that drunk,” said Cpl. Anthony Bertagna. “I’m surprised she was still functioning.”

Facts on Tap, a national alcohol-education program says that someone with a blood-alcohol level of .30 is “in a stupor” and at risk of suddenly passing out. It’s approaching the level of surgical anesthesia; concentrations just a few percentage points higher have actually killed people.

That kind of breath test is “extremely accurate,” Bertagna said, but it’s not official. Officers arrested the Placentia resident on suspicion of driving under the influence; she was booked into jail, where her blood was taken for an additional alcohol test. The results of that test were not back by Friday afternoon.

Court records show that a Leann Arriaga with her birth date has been ticketed six times in Orange County in the past five years for speeding and other traffic infractions, as well as littering. Other officers have occasionally pulled people over with that level of alcohol in their blood, he said, but they’ve never seen anyone that drunk and still behind the wheel.